1. Technical Field
This invention relates to a method for preparing an optical recording medium by applying a dye solution to form a dye film, and an optical recording medium.
2. Background Art
In recent years, various optical recording discs of the write-once, rewritable and other types draw attention as high capacity information carrying media.
Among the optical recording discs, there are known those having a dye film composed mainly of a dye as the recording layer. From a structural aspect, optical recording discs proposed thus far include widespread discs of the air-sandwich structure having an air space on a dye film and discs having a recording layer made of a dye film which can be read in accordance with the compact disc (CD) standard and a reflective layer disposed in close contact therewith. (See Nikkei Electronics, Jan. 23, 1989, No. 465, page 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science and Technology Center; and Proceedings SPIExe2x80x94The International Society for Optical Engineering, Vol. 1078, pages 80-87, xe2x80x9cOptical Data Storage Topical Meetingxe2x80x9d, Jan. 17-19, 1989, Los Angels.)
The dyes used in such recording layers include cyanine dyes, phthalocyanine dyes, and azo metal dyes.
In general, recording layers are formed from these dyes by dissolving the dyes in organic solvents and applying the solutions by a spin coating technique. The spin coating technique is advantageous in that it is simple and does not require a large equipment. The spin coating technique, however, suffers from a low production yield. Since dyes are expensive, the low utilization of dyes causes the cost to increase due to a loss.
The requirement on the solvents of dye solutions is that dyes are fully soluble therein. In the prior art, keto-alcohol, ketone, alcohol, hydrocarbon, ester, ether, cellosolve, halogenated alkyl, and halogenated alcohol solvents are used. Among others, halogenated alcohols, especially fluorinated alcohols are advantageously used because of the high solubility of dyes therein.
In forming a dye film by coating, the utilization of a dye solution may be increased by reducing the discharge amount of the dye solution. However, if the discharge amount is below a certain level, the substrate surface is not fully covered with the solution, resulting in coating defects, for example, leaving an uncovered portion in a comb shape along the periphery of the optical recording disc. When spin coating is carried out using the above-mentioned solvents, the discharge amount must be increased in order to produce optical recording media of quality and free of such coating defects. This results in a substantial loss and low utilization of the dye solution. In the case of continuous coating, the discharge pressure at which the coating solution is discharged from a nozzle becomes high, resulting in variations of film thickness and occurrence of coating defects.
An object of the invention is to provide a method for preparing an optical recording medium, featuring a high production yield, a reduced loss and a high utilization of dye solution, and ensuring to produce media of consistent quality even by continuous coating as well as an optical recording medium.
Making extensive investigations to solve the above problems, the inventors have found that when a main solvent in which a dye is fully soluble is combined with a second solvent having a hydroxyl group (xe2x80x94OH group), the wetting of a substrate with the solution is improved so that the discharge amount of the dye solution can be reduced. The present invention is predicated on this finding.
The above and other objects are achieved by the present invention which is defined below.
(1) A method for preparing an optical recording medium, comprising the step of applying a solution of a dye in a solvent onto a resin substrate to form a dye film, wherein
the solvent of said dye solution is a mixture of tetrafluoropropanol and/or octafluoropentanol with an aliphatic saturated alcohol of 1 to 4 carbon atoms, and
a recording layer is formed by applying the dye solution.
(2) A method for preparing an optical recording medium according to (1) wherein the content of the aliphatic saturated alcohol is 0.5 to 40% by weight of said solvent.
(3) A method for preparing an optical recording medium according to (1) wherein said dye has a solubility of at least 0.3% by weight in tetrafluoropropanol and/or octafluoropentanol at 25xc2x0 C.
(4) An optical recording medium prepared by the optical recording medium preparing method of (1).